Capacity for Digestion and Effects of Composition of Absorbed Nutrients on Animal Metabolism

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The capacity for ruminal fermentation and postruminal digestion have been briefly discussed. From the available data it would seem that, provided the diets are highly digestible and easily degradable, the capacity for rumen fermentation far exceeds the energy needs of the host ruminant animals, though, as is well known, this is not so with many roughage diets. For most types of animal production, microbial protein and undegraded dietary protein provide sufficient or excess protein for the need of the host animal. For some systems, however, (e.g., early weaned lambs and calves and high producing dairy cows) the need is not met and protein must be given, which in order to be effective, must at least partly escape the rumen undegraded. The capacity for postruminal digestion would probably also be sufficient to meet the energy needs of the growing ruminant. Severe constraints are imposed on diet composition if the method is used alone; the diets could not consist of carbohydrates and protein alone but would have to include substantial quantities of lipids. Apart from influencing fermentation losses (see previous papers) the composition of energy yielding nutrients abosrbed, expressed as proportions of glucose or glucose precursors, can influence host animal metabolism in two distinct ways. It can influence the obligatory heat losses associated with metabolism. It can affect the partition of energy into different types of animal products. Possibilities of using both the ruminal and postruminal capacity for digestion to achieve the optimum quantity and composition of absorbed nutrients for different types of production have been discussed. It is concluded that there is a pressing need for more information which will enable nutritionists to define more clearly the optimum composition of absorbed nutrients for different types of production in order to achieve the most efficient utilization of energy, and — possibly more important — to achieve the optimum proportions of desired products.